Something to watch for in the new data from the Large Hadron Collider

A new result released by the ATLAS experiment at CERN shows an intriguing anomaly, which could be evidence for a new particle with a mass of about two thousand times the mass of a proton. How excited should be we be?

Display of one of the proton-proton collisions containing W and Z boson candidates. The signals from the Z candidate jet are marked in red, while the signals from the W candidate jet are marked in green. The WZ combination has a reconstructed mass of 2.26 TeV
Photograph: ATLAS/CERN

I’m going to try to walk a line here, between hyping a result and being overly conservative. Let’s see how it goes.

As has been widely reported, the Large Hadron Collider at CERN is in operation again, colliding particles at a higher energy of 13 tera-electron-Volts (TeV), compared to 8 TeV in 2012. Having discovered the Higgs boson - the last particle predicted by the “Standard Model” of particle physics, hope and fears abound as to what might be revealed by the data now being collected.

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At the back of our minds is the fear that, as we study physics at these unprecedented energies - which means unprecedentedly high resolution, probing finer structure of the universe than ever studied directly before - “all” we will do is find out that the Standard Model works well in this regime, that the Higgs boson we have found looks just like the Standard Model Higgs boson, and that all the questions that the Standard Model leaves unanswered (and there are many!) remain unanswered. It is even legitimate to ask whether the LHC might see the end of particle physics as we know it.

The minimalist, pessimist’s approach to this dilemma involves trying to push the Standard Model to ridiculously high energies to see where it must break down. Pre-Higgs, this approach worked well for the LHC (it was the approach we took in my first paper on LHC physics, back in 2002). Unfortunately, now that the Higgs has been discovered, similar arguments indicate that the Standard Model seems to remain internally consistent up to energies well beyond the reach of current or foreseeable particle colliders. This means we’re flying blind. We know physics beyond the Standard Model is out there, but there is no guarantee it lies within the new terriory we are now exploring. It is exciting, but a little nerve-wracking.

Under such tension, any sign of a new particle, or any other interesting deviation from the predictions of the current theory, is bound to cause a stir. Just as data-taking began, my experiment, ATLAS, published some data from the 8 TeV run which show signs of just such an effect.

During the Higgs boson search, the most important data we were looking at were mass distributions. These distributions are a good way of testing the hypothesis that a new particle has been produced somewhere in a collision. We look at the particles that are actually measured in the detector, and we ask the question

If these particles came from the decay of a new, to-date undiscovered particle, what would its mass be?

This question can be answered - with a bit of relativistic kinematics, the mass can be reconstructed. If there is no new particle, the distribution of reconstructed masses won’t show any special features. But if a new particle is being produced, and produced often enough, there will be a concentration of data with reconstructed masses close to the mass of the new particle - a bump. It was bumps in this kind of distribution, at a mass of around 125 GeV (about 125 times the mass of the proton¹) that showed the existence of the Higgs boson.

Below is another mass distribution. This one is constructed by identifying “jets” of hadrons which look like they come from the decays of W and Z bosons (the carriers of the weak nuclear force), and evaluating the mass of a particle that might have produced them - the “diboson mass” spectrum. The intriguing question is, can you see a bump in it?

Measured distribution of the dijet mass after selecting WZ candidates. The data (black points) are compared to the expected background (blue line). The signals expected for a new W’ boson with masses of 1.5, 2.0, or 2.5 TeV are displayed for illustration purposes Photograph: ATLAS/CERN

Well, actually the answer to that question is “yes”. There is a bump at about 2 TeV (2000 or so proton masses). But as we saw during the Higgs search, bumps and hints can come and go. The important question is, what is the significance of any bump in that distribution? This means trying to evaluate to probability that the data are consistent with the Standard Model plus some random statistical noise.

The local statistical significance is shown in the bottom inset in the plot, in blue. The combined significance of the deviation of the three points near 2 TeV is 3.4 σ (3.4 sigma) which, naively, corresponds to a pretty small probability - about 0.03% - that the Standard Model would randomly produce such a bump. “Three sigma” local significance like this is traditionally called “evidence” for something in particle physics (with “five sigma” needed to claim a discovery).

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So by that measure, these data constitute evidence for a new particle. However, I did use the word “naively” above, and there is a catch. Physicists at the LHC are simultaneously studying thousands of distributions, and if you study enough of them, very likely some of them will show rare fluctuations like this. So how should we evaluate the strength of the evidence?

As with the Higgs search, ATLAS and CMS try to account for this by evaluating what we call a “look elsewhere” effect. With the Higgs bump hunt, this was fairly simple. We were looking over a particular range of masses, and we could ask the question

What is the probability that we would get a bump like this anywhere in this mass range if there were no Higgs?

This will typically give a higher probability - and so a less significant result - than the answer to the question

What is the probability that we would get a bump like this right here at this massif there were no Higgs?

In this new analysis, we evaluated the probability of a bump like this showing up anywhere in the diboson mass spectrum. As expected, this comes out lower, at 2.5 σ. This is below the conventional threshold for “evidence”, although it’s worth remembering that that threshold is usually applied to the local significance, before the look-elsewhere effect is accounted for.

But the diboson mass distribution is just one of many distributions we’re studying, looking for anomalies. If you took them all into account you could reduce the significance further. In the end (to the annoyance of many) it is impossible to remove an element of judgement, of subjectivity. In Bayesian language, something like a prior assumption.

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Stepping back a moment to see what is at stake here; a new particle at 2 TeV would be tremendously exciting. It is absolutely not expected in the Standard Model. It would mean the new territory being explored by the LHC is not a desert, but contains measurable, new, fundamental physics. This would wonderful to explore in itself, and it would be very reasonable to expect it to give clues, or even answers, to some of the open questions about the universe, such as the nature of Dark Matter, the seemingly unfeasible coincidence of the Higgs mass being so low, or the fact that there is so much more matter than antimatter in the universe. It would also put experiment firmly ahead of theory in probing these questions, which is something both experimentalists and theorists (sometimes secretly) long for.

If you ask me, I take this bump seriously enough to be very interested, bordering on excited, but if I had to bet I would bet against it being a new particle. Then, I’ve always been a pessimist on these things - I confess to a very strong prior in favour of the Standard Model winning.

In the end, however seriously or not one takes this anomaly, the only way to know for sure is take more more data. Which we are now doing. And we don’t have a “look elsewhere” effect anymore for this anomaly. We’ll be looking at 2 TeV in the diboson mass spectrum. A definite, intriguing and exciting case of “watch this space”.

¹Fans of units might wonder at measuring mass in GeV or TeV, which are units of energy. You might expect it to be GeV/c² etc. I (and ATLAS) are using a particle physics unit system called “natural units” in which the speed of light c=1, so doesn’t need to be written there.

Note added 29/6/15 it is worth noting, still without getting too excited, that the CMS experiment have also measured this distribution and see a smaller, but apparently consistent, bumplet.

Jon Butterworth’s book Smashing Physicsis available as “Most Wanted Particle” in Canada & the US. He is also on Twitter.